This function returns via the provided callback.  In the normal case, the first
argument will indicate success or failure of the operation with one of the
following values:

\begin{itemize}[noitemsep]
\item A Javascript representation of the stored object.
\item \code{null} if the operation is a retrieval operation and no object was
    found.
\end{itemize}

The second argument is a true/false value indicating whether the operation is
complete.  The callback will be called repeatedly until all objects are
returned or encounter errors.

If the operation encounters any error, the error argument will be provided and
will specify the error, in which case the first argument is undefined.
